In a network with a plurality of terminals and a plurality of switches connected to one another via data lines, a multicast data packet is transmitted according to the MMRP protocol from one data source via at least one switch and at least one of the data lines to a predetermined terminal intended to receive the multicast data packet.
In contract, a unicast packet is a data packet addressed to a single predetermined terminal and relayed by switches thereto in a targeted manner. Multicast packets are data packets relayed to a plurality of receivers within a network. A switch according to standard IEEE 802.1D in the basic setting always relays multicasts to all ports of the switches, but not to the ports of those terminals that have already received the packet. Therefore, such packets are also relayed into regions of a network where they only senselessly consume the available bandwidth.
The multiple MAC registration protocol (MMRP) and its precursor GMRP (GARP multicast registration protocol) are used to relay multicast packets in a targeted manner only to those terminals that are of interest for its data stream or for which they are intended so as not to waste bandwidth in other regions of the network. This is optimal as long as all terminals support the MMRP protocol or the GMRP protocol. Hereafter, only “MMRP” is used, but applies also to GMRP (or vice versa, also with respect to the illustrations in the drawing).
In industrial networks, terminals often are and will be used that do not support MMRP. As a result, they cannot register for a specific multicast stream in the case of MMRP-activated switches and therefore also do not receive the multicast stream related thereto.
If no terminal is registered for a multicast, the switch behaves as if MMRP were deactivated or not present. It thus relays incoming multicasts to all other ports. However, this state is not of further interest and is only mentioned here for the sake of completeness.
Thus, if a terminal in an MMRP environment does not support the MMRP standard (computer 2 in FIG. 1 as an example), there are then two possibilities so that this terminal can receive a specific multicast stream:
Possibility 1:
The port of the switch to which the non-MMRP-capable terminal is connected must receive a static entry for this multicast. This is also true for the port of the following switch, to which the first switch is connected. This continues up to the source of the multicast.
This means, when considering FIG. 2, that the port 3 must be statically configured by switch A, and also the port 1 must be statically configured by switch B.
Disadvantages of possibility 1:
Every switch between the sender and receiver must be configured. Switches introduced later (that were placed in the network newly or after a replacement, for example) also cannot be forgotten. This is very susceptible to error.
These configurations are static. If the multicast is no longer used, the affected switches must be reconfigured again.
The more different multicasts are required, the more complex the configurations.
Possibility 2:
At the port to which the terminal is connected, and at all further “input ports”, “forward all” can be set. All multicasts are then relayed to the terminal. This terminal would guide the multicast control ad absurdum.
Disadvantages of possibility 2:
Multicasts that are not required are relayed uselessly consume the bandwidth of the terminal and unnecessarily load the terminal CPU.
The entry is static on all terminals between sender and receiver, as in possibility 1. Changes correspondingly ensure greater effort in the configuration.